1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display and a method for fabricating the same, and more particularly, to an OLED display and a method for fabricating the same in which an organic capping layer having a refractive index of 1.7 or more is formed on a second electrode of the OLED display.
2. Description of the Related Art
Among flat panel displays, an organic light emitting diode (OLED) display is an emission display that emits light by electrically exciting an organic compound. As used herein OLED display may refer to a single OLED, multiple OLEDs or an OLED display device. The OLED display requires no backlight, which is used in a liquid crystal display (LCD). Accordingly, the OLED display can be formed to be light-weight and compact in a simplified process. The OLED display can be fabricated at low temperatures and has a fast response speed of 1 ms or less and low power consumption. Further, the OLED display has a wide viewing angle and a high contrast ratio because of its self-emission feature.
Typically, since an OLED display includes an organic emission layer between an anode and a cathode, holes supplied from the anode and electrons supplied from the cathode are recombined in the organic emission layer, thereby creating hole-electron pairs, i.e., excitons, which generate energy when being transitioned to a ground state to emit light.
An OLED display may be classified into a bottom-emitting OLED display and a top-emitting OLED display depending on an emitting direction of light that is generated from an organic emission layer. In the bottom-emitting OLED display in which light is emitted toward a substrate, a reflective electrode is formed on an organic emission layer and a transparent electrode is formed beneath the organic emission layer. Here, if the OLED display is an active matrix one, a portion with a thin film transistor formed therein does not transmit light so that a light emitting area is reduced. On the other hand, in the top-emitting OLED display, a transparent electrode is formed on an organic emission layer and a reflective electrode is formed beneath the organic emission layer so that light is emitted in an opposite direction to a substrate side, thereby increasing a light transmitting area and in turn improving luminance.
FIG. 1 is a cross-sectional view showing the structure of a conventional top-emitting OLED display.
Referring to FIG. 1, in the conventional top-emitting OLED display, a first electrode is patterned and formed over a transparent substrate 11 such as glass or plastic. The first electrode includes a reflecting layer 12 formed of a metal having a high reflectivity characteristic such as aluminum (Al) or aluminum-neodymium (Al—Nd). The first electrode is formed of a reflective electrode that further includes a transparent electrode 13 such as Indium Tin Oxide (ITO) or Indium Zinc Oxide (IZO) having a high work function.
An organic layer 15 including at least an organic emission layer is then formed on the first electrode 13 and may further include at least one of a hole injection layer, a hole transport layer, an electron transport layer, and an electron injection layer, in addition to the organic emission layer.
Then, a second electrode 17 is formed on the entire surface of the organic layer 15. The second electrode 17 is formed of a conductive metal having a low work function selected from Mg, Ag, Al, Ca and alloys thereof. The second electrode 17 may be formed of a transmissive electrode with a small thickness such that light is transmitted or of a transparent electrode such as ITO or IZO.
A passivation layer 19 is formed on the second electrode 17. The passivation layer 19 may be formed by stacking an inorganic material or an organic material. In one embodiment, the passivation layer 19 is formed to a certain thickness using an inorganic layer such as a silicon nitride (SiNx) layer to protect the second electrode and the organic emission layer from external moisture and contamination.
In a conventional OLED display, the passivation layer is formed of an inorganic or organic material on the second electrode to protect the second electrode and the organic emission layer from external moisture and contamination after the second electrode, which is a transmissive electrode, is deposited. However, as the passivation layer is deposited without any consideration of its refractive index and thickness, the top-emitting OLED display degrades in efficiency.